Position the top rack four to five inches from the top of the oven. Take the steak out of the refrigerator as you pre-heat the broiler to its highest possible temperature (Our broiler reaches to 550 degrees). Open a window just in case. Things might get a little smoky.
While the broiler warms, pre-heat an empty cast-iron skillet or grill pan on high heat on the stove for about five minutes. If you're using a premium cut of aged meat and a well-seasoned pan, you won't need to add any oil to the pan. The screaming hot pan might just start to smoke a bit. Be careful and be sure to use a thick oven mitt.
Once both the broiler and the pan are pre-heated, sprinkle the steak with salt and put it in the pan (We don't put black pepper on until after it comes out of the broiler because we think it can burn, but others differ with us on this). Place the pan on the top rack, under the broiler.
Let the steak broil for four minutes and then carefully flip it to the other side for another four minutes. If you're using a grill pan with raised ridges make sure you don't wiggle the steak around in the pan. You'll mess up the grill marks.
After the steaks have broiled on each side, pull the pan out of the broiler. Use a paring knife to make a small cut in the center of the larger side of the Porterhouse see if the steak is done to your liking. Remember, the steak will keep cooking a bit even after it is out of the broiler. (Don't feel too bad about cutting into the steak. Yes, you will let some of the juice out of the steak, but losing some juice is better than over-cooking the steak.) If you'd like the steak more well done, return it to the broiler, checking it again after another minute.
Move the steak from the pan to the platter and serve immediately.
We came to this method after making a whole bunch of steaks and consulting with at least four butchers, The Joy of Cooking, and Mark Bittman's How To Cook Everything.
I like cooking thermometers, too, if you can't tell by feel how well a slab of meat is cooked:
150 Medium Rare
165 Medium Well
And I've learned and found that the temp of a steak will rise 5 to 10 degrees, depending on thickness, while resting.
I tend to let meat rest a little after it's done cooking to let the juices redistribute and let the carryover heat finish cooking the steak. It's not as big a deal with such a small chunk of meat, but I've found that 10 minutes under a foil tent can make a difference.